The linguistic phenomenon of polyfunctionality, in which a from can perform more than one syntactic function and can denote different yet related meanings, is of particular interest from the linguistic typological, historical linguistic, and cognitive linguistic points of view. This study primarily focuses on the form /tôŋ/ in Thai, which constitutes an interesting instance of polyfunctionality as its counterparts are found to be prevalent in the Southeast Asian languages. However, it is not known how the multiple functions of /tôŋ/ are historically related as well as conceptually associated. To answer this research question, this study is thus aimed to analyze the syntax and semantics of /tôŋ/, trace the path and direction of its grammaticalization and semantic extension, and identify the mechanisms that trigger these changes. It is found that the form /tôŋ/ has two functions, the verb and auxiliary function, which can be distinguished from each other by the criteria of propositionality, distribution, control, and negation. Accordingly, the eight meaning of /tôŋ/ can be categorized into the lexical senses, which include 'coming into physical contact," "being in correspondence," "being subject to a supernatural influence," and "receiving a social obligation," and the modal meanings, which include "having an obligation to do something," "having a necessity to do something," "having a need to do something," and "having a certainly to do someting." There are six stages through which /tôŋ/ develops from its verb function to its auxiliary function, and reanalysis and analogy are mechanism responsible for this grammaticalization, though at different stages. Moreover, the semantic extension of /tôŋ/ can be broken down into three paths, and the cognitive mechanisms of metaphor and metonymy are both involved, though at different stages.